Phosphoglucoisomerase



Phosphoglucoisomerase (alternatively known as phosphoglucose isomerase or Glucose-6-phosphate isomerase) are a group of enzymes of the isomerase family (EC 5.3.1.9), so named for their main function in glycolysis and gluconeogenesis. In both these pathways phosphoglucose isomerase (PGI) is used to inter-convert glucose-6-phosphate and fructose 6-phosphate. This reaction is driven by the relative concentrations of these sugars in the cytoplasmic matrix of the cell. The overall reaction can be seen here.

Phosphoglucoisomerase is also known for a list of activities:
 * Neuroleukin (NLK)- nerve growth factor. Secreted by T cells, promotes the survival of certain sensory and embryonic nerve cells. Also used to stimulate the production of immunoglobulin.
 * Autocrine motility factor (AMF)- product of tumor cells, it promotes cell migration and viewed as a possible cause in cancer metastasis.
 * Maturation factor(MF)
 * Myofibril-bound serine protese inhibitor (MBSPI)
 * PGI is important for metabolism in many different clades, including eukarya, bacteria, and archea..
 * Involved in Gluconeogenesis in which it catalyzes the reaction of D-glucose to D-Fructose

Structure


Phosphoglucose isomerase exists in the cell usually as a homodimer, nevertheless outside of the cell, it has been isolated as a monomeric structure. PGI has essentially an identical fold in all of the characterized species (see Figure 1). The secondary structure of phosphoglucose isomerase is charaterized by an αβα conformation, on each of its two domains. The smaller domain is characterized by 5 parallel β-sheets, while the larger domain if formed out of 6 parallel/antiparallel β-sheets. Furthermore, another characteristic trait is a residue extension at the C-terminus, which wraps around the other monomer in the dimeric conformation. A "hook" that can potentially be involved in the previously mentioned extracellular activities.

Phosphoglucose isomerase has a monomer molecular mass of proximately 55 kDa.

Active Site - Mammalian PGI shows a degree of conservation ( dark red for highly conserved regions - dark blue for variable reigions) of about 90 %. The active site is the region with highest observed conservation, containing a number of residues that are crucial in the enzyme-substrate interaction mechanism (Lys210, Gln353, Glu357, Gln511, Lys518, His388b). Lys518(His388) and Glu357 Lys 518(His388) and Glu357 are the main components of ring opening, while many of the other residues can be used for stabliziation and orientation.

Another characteristic of phosphoglucose isomerase is that binding of substrate at the active site induces a small movement in the conformation of the enzyme. This can be seen in Figure 2 as change in the position of an α helix.

Mechanism
The proposed reaction mechanism of PGI for the reversible conversion of glucose-6-phosphate to fructose 6-phosphate involves an acid/base catalysis by the enzyme. The basic mechanism involves the isomerization of an aldose to a ketose. This is performed by a ring opening, followed by an isomeration of the opened ring, then a ring closing. A detailed step by step mechanism of this process can be seen as follows :

Step 1. The substrate binds to the enzyme.

Step 2. The residue Lys518 (or His388b) acts as an enzymatic acid catalyzing the opening of the ring.

Step 3. Conserved Glu357 abstracts the acidic proton from C2 forming a cis-enendiol intermediate.

Step 4. Glu357 donates back the proton at the C1 position.

Step 5. Lys518 (or His388b) abstracts back the proton from the sugar ring oxygen, resulting in a ring closure, to give the product.



Regulation and Inhibition
Regulation of phosphoglucoisomerase is only done by the relative concentrations of glucose-6-phosphate and fructose 6-phosphate, towards equilibrium. Nevertheless, it was found that the kinetic parameters of PGI does depend on the pH and temperature of the environment. The following kinetic parameters are proposed for rabbit PGI at pH 8.5 and 30°C



It is interesting to point the regulation of PGI in other aspects that are not involved in metabolism. For example, PGI acts as a "cytokine" outside the cell in that it can be used as a cell signalling protein. PGI has been found to to be associated with AMF cells, which is found to regulate tumor cell motility. Regulation of these extracellular "cytokine" PGI/AMF can be seen. The amount of PGI/AMF that is secreted inside and outside the cell based on infection.

Inhibition   of the phosphoglucoisomerase regulated reaction of glucose-6-phosphate to fructose-6-phosphate can also occur. Competitive competition can take place from inhibitors such as 5PAH. 5PAH resembles PGI, differing only in a nitrogen atom at the first carbon position. 5PAH is reported to have a Ki of .0000002 M.

Links

 * Crystal Structure of human phosphoglucose isomerase (PDB=1iat)


 * Crystal Structure of rabbit phosphoglucose isomerase complexed fructose 6-phosphate (PDB=1hox )


 * Other available structures 1dqr, 1g98, 1gzd, 1gzv, 1hm5, 1iri, 1jiq, 1jlh, 1koj, 1n8t, 1nuh, 1xtb.

Additional Resources
For additional information, see: Carbohydrate Metabolism